There are two basic ways in which animals maintain homeostasis with respect to water and electrolytes. Some animals match their internal osmotic pressure to that of their external environment: These animals are osmoconformers. They keep their internal fluids at the same osmotic pressure as the surrounding environment, which reduces the movement of water and solutes into or out of their bodies. Because the solute concentrations inside and outside osmoconformers are similar, osmoconformers don’t have to spend a lot of energy regulating osmotic pressure. However, they have to adapt to the solute concentration of their external environment. Osmoconformers tend to live in environments like seawater that have stable solute concentrations because maintaining stable internal solute concentrations is easier in stable environments.

Although osmoconformers generally match the overall concentration of their tissues with their external environment, they often expend energy to regulate the concentrations of particular ions such as sodium, potassium, and chloride and of other solutes such as amino acids and glucose. For example, whereas the intracellular space of nearly all multicellular animals has a relatively high concentration of potassium ions and low concentration of sodium ions, the extracellular space has concentrations that are just the opposite—low in potassium and high in sodium. This means that these cells are actively pumping sodium out and potassium in (Chapter 5).

Most marine invertebrates, such as sea stars, mussels, lobsters, and scallops, are osmoconformers, matching their total intracellular solute concentration to the solute concentration of seawater. These organisms maintain high concentrations of sodium and chloride in their cells to achieve an overall solute concentration close to that of seawater. Consequently, they have few specializations for osmoregulation beyond the need to regulate specific internal ion concentrations relative to their environment. Some marine vertebrates are also osmoconformers. Hagfish and lampreys, for example, match their total internal solute concentration to that of seawater by maintaining high internal concentrations of particular electrolytes, just as marine invertebrates do.

Other osmoconformers, like sharks, rays, and coelacanths, match seawater’s solute concentration by maintaining a high internal concentration of a compound called urea. Urea is a waste product of protein metabolism that many animals excrete, so it does not build up to high concentrations (section 41.2). By retaining this solute, these marine vertebrates are able to achieve osmotic equilibrium with the surrounding seawater.